The noise reduction microphone is a microphone that uses direction to process the sound signal to improve the signal-to-noise ratio, so that the user can hear the opposite person when there is background noise. Mike, whose scientific name is Mike, is translated from English microphone (microphone), also called Mike, microphone. A microphone is an energy conversion device that converts sound signals into electrical signals. Categories include actuators, condensers, main pole bodies, emerging silicon microphones, liquid microphones, laser microphones, etc. Most microphones are electret condenser microphones, which work by using a high-molecular diaphragm with charge isolation.
The basic principle of noise reduction microphones is that according to strict acoustic principles, two microphones are assembled on the same hearing aid. The sound signals arriving from different angles are amplified differently to enhance useful signals and relatively reduce background noise.
Most noise reduction microphones are electret condenser microphones (ECM), which have been used for decades. The working principle of ECM is to use a polymer material diaphragm with permanent charge isolation. MEMS microphones are not affected by temperature, vibration, humidity and time, and are compared with ECM's polymer diaphragms at different temperatures. Due to the strong heat resistance, the MEMS microphone can withstand high temperature reflow soldering at 260C without changing its performance. The sensitivity change before and after assembly is small, so audio debugging costs can be reduced during the manufacturing process. At present, integrated circuit technology is more and more widely used in the manufacture of sensors and sensor interface integrated circuits. The micro-manufacturing process has the advantages of high precision, flexible design, miniaturization, integration with signal processing circuits, low cost, and mass production.
The research results show that the early miniature microphones were based on the piezoresistive effect, made of (11) cm2, 2m thick polysilicon film made of sensitive film microphones. However, if there is no stress in the sensitive film, the first resonance frequency of such a large and thin polysilicon film will be below 300 Hz. In a frequency range as low as the first resonance frequency, the frequency response of the microphone's auditory frequency range may become very uneven (the sensitivity change exceeds 40dB), so the microphone application is unacceptable. When the sensitive film has tensile stress, the resonance frequency will increase, but at the expense of sensitivity. Of course, the size of the sensitive film can be adjusted to obtain a higher primary resonance frequency, but the sensitivity will still be reduced. It can be seen that the suppression scheme is not suitable for manufacturing miniature microphones.
One possible solution is to use a capacitive solution to make a miniature microphone. The advantage of this method is that all materials used in the integrated circuit manufacturing process can be used for sensor manufacturing. However, it is quite difficult to make a miniature microphone with a single-chip technology. Because the air medium between the two capacitor plates can only have a small gap. In addition, due to size limitations, the offset voltage of some applications is not satisfactory. Based on these issues, the research on condenser microphones has not been interrupted.
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